Thin film formation through sputtering comprises introducing a rare gas such as argon into a vacuum chamber, applying a negative voltage to the sputtering target in the chamber to cause glow discharge therein, making the thus-formed plasma cations collide with the sputtering target, and depositing the thus-released, sputtering target-constitutive atom on the substrate disposed opposite to the target.
Typically, the sputtering target includes aluminium, aluminium alloy, high-melting-point metal (e.g., tungsten, molybdenum, titanium) and its alloy, high-melting-point silicide (e.g., molybdenum silicide, tungsten silicide); and recently, a sintered ITO (indium-tin-oxide) body has become used for it.
A backing plate functions as a support, and further functions as a cooling medium for removing heat from the sputtering target that generates heat therein during sputtering. Accordingly, the backing plate generally has a cooling mechanism inside it.
For the backing plate, used are oxygen-free copper, copper alloy, aluminium alloy, stainless steel, titanium alloy, etc. Of those, a copper backing plate is favorably used, as its thermal conductivity is good. However, its linear expansion coefficient is 17×10−6/K and is large, and this is problematic in that its deformation is great owing to its elongation in repeated use and, after used a few times, it could no more be reused.
The sputtering target for use in the above-mentioned sputtering step is generally used in the form of a structure thereof soldered to a backing plate with a low-melting-point soldering material.
In the sputtering step, when the sputtering target structure is heated high owing to the collision of plasma cations therewith and when there is a great difference in the linear expansion coefficient between the sputtering target and the backing plate constituting it, then the sputtering target structure may deform like a bimetal. The deformation may occur also in producing the sputtering target structure.
The deformation produces some problems in that the sputtering target may crack, the soldering material may fuse and the soldered part may peel.
Accordingly, a sputtering target structure is desired, which does not cause bimetal deformation even when exposed to high temperatures in sputtering and which is therefore free from the problems of cracking and peeling of the sputtering target thereof.
Recently, molybdenum and titanium have become used as a sputtering target material, and the linear expansion coefficient of these metals greatly differs from the linear expansion coefficient of copper that is favorably used for a backing plate material. Therefore, the requirement for the sputtering target structure free from the above-mentioned problems is great.
For obtaining the sputtering target structure free from the above-mentioned problems, various proposals have heretofore been made.
For example, investigations of soldering materials and soldering methods have produced a method comprising disposing a tin alloy sheet or the like between a sputtering target and a cooling plate (backing plate), putting an indium alloy between the sheet and the sputtering target and between the sheet and the cooling plate, and heat-sealing them (Patent Reference 1); a method comprising subjecting a sputtering target for plating to a pretreatment such as plating or vapor deposition thereon and inserting a buffer material between the sputtering target and a cooling plate (backing plate) (Patent Reference 2); a method comprising inserting an insert material between a sputtering target and a cooling plate (backing plate) (Patent Reference 3); a method comprising inserting a heat-conductive adhesive between a sputtering target and a backing plate and adhering them with a low-melting-point metal disposed between the adhesive and the sputtering target and/or the backing plate (Patent Reference 4).
These methods are to attain the effect through modification of the constitution of the thin solder material layer between the sputtering target and the backing plate, and they could hardly attain the intended effect. In these methods, in addition, the minus effect of thermal conductivity depression owing to the disposition of various materials between the sputtering target and the backing plate cannot be bypassed. Further, the use of indium is problematic in point of its cost.
On the other hand, a backing plate investigation has proposed a method of forming a three-layered backing plate structure of molybdenum-titanium-molybdenum, as is disclosed in Patent Reference 5. This is for making the linear expansion coefficient of the backing plate the same as that of the sputtering target to be combined with it.
Patent Reference 6 discloses a backing plate formed of a composite material of molybdenum and copper.
Patent Reference 7 proposes production of a backing plate from a composite material of aluminium or aluminium alloy with ceramic.
However, these materials have poor machinability, and are hardly applicable to large-sized products for FPD for which, in particular, the thermal expansion difference between the sputtering target and the backing plate is especially problematic.
Patent Reference 8 proposes a backing plate comprising titanium. However, its thermal conductivity is extremely low and this is unfavorable to a structure of a sputtering target and a backing plate equipped with a cooling mechanism.
Further, Patent Reference 9 proposes a technique of providing grooves on the surface of a backing plate to be bonded to a sputtering target. In this, the grooves are to absorb thermal deformation, but on the other hand, this is problematic in that its thermal conductivity depression is inevitable.    Patent Reference 1: JP-A 61-250167    Patent Reference 2: JP-A 5-25620    Patent Reference 3: JP-A 4-365857    Patent Reference 4: JP-A 2000-160334    Patent Reference 5: JP-A 8-246144    Patent Reference 6: JP-A 62-67168    Patent Reference 7: JP-A 2002-161361    Patent Reference 8: JP-A 6-293963    Patent Reference 9: JP-A 2-43362